Industrial Ethernet Introduction
l Introduction to Ethernet and Industrial Ethernet
This paper analyzes the network communication mechanism of Ethernet and points out the problems that Ethernet has in entering industrial communication networks and some of its solutions. Then a detailed description of a widely used industrial Ethernet (SIMATIC NET) in the current industrial communication network.
The emergence of fieldbus has played a huge role in promoting the realization of the equipment-oriented automation system. However, such dedicated real-time communication networks as fieldbuses have the disadvantages of high cost, low speed, and limited support applications, coupled with the diversity of bus communication protocols. Therefore, different bus products cannot interconnect, interoperate and interoperate with each other, so the further development of the fieldbus industrial network is greatly limited. With the development of Ethernet technology, especially the emergence of high-speed Ethernet, Ethernet can overcome its own shortcomings and enter industrial fields as industrial Ethernet, thus enabling people to use Ethernet equipment instead of expensive industrial network equipment.
l The main flaw of Ethernet
Before talking about the main drawbacks of Ethernet, it is necessary to understand the Ethernet communication mechanism. Ethernet refers to a network that complies with the IEEE 802.3 standard and can be transmitted over fiber optic cables and twisted pair cables. It first appeared in 1972 and was created by Xerox PARC. At present, Ethernet adopts star and bus type structure, transmission rate is 10Mb/s, 100Mb/s, 1000Mb/s or more. The main reason for Ethernet delay is collision, which is because it uses CSMA/CD technology. In a traditional shared network, because all the sites in the Ethernet are connected using the same physical medium, this means that when two devices transmit signals at the same time, signal conflicts will occur. In order to solve this problem, Ethernet stipulates that before a site can access media, it must first monitor whether other sites on the network are using the media at the same time. If you do, you must wait and a conflict occurs. In order to reduce the chance of collisions occurring, Ethernet often employs 1-persistent CSMA, non-persistent CSMA, and P-persistent CSMA algorithm 2 . Because Ethernet is designed for office automation, it does not fully comply with the requirements of the industrial environment and standards. There are still obvious drawbacks to using traditional Ethernet in the industrial field. However, its cost is lower than that of industrial networks, and its technical transparency is high. In particular, it follows the IEEE802.3 protocol to open the door for field bus manufacturers. However, if Ethernet is to meet the process requirements, the following defects must be overcome:
l Certainty
Since the MAC layer protocol of Ethernet is CSMA/CD, this protocol makes a conflict on the network, especially when the network load is too large. For an industrial network, if there are a lot of conflicts, the data must be retransmitted many times, making the uncertainty of the inter-network communication greatly increase. This kind of uncertainty from one place to another in an industrial control network will inevitably bring about a reduction in the control performance of the system.
l Real-time
In an industrial control system, real-time can be defined as the testability of the system's reaction time to an event. That is, after an incident occurs, the system must respond within a time frame that can be accurately predicted. However, the real-time requirements for data transmission in the industry are very strict, and often the data update is completed in tens of ms. However, due to the CSMA/CD mechanism that exists in Ethernet, when there is a conflict, data must be retransmitted, up to a maximum of 16 attempts. It is clear that this mechanism for resolving conflicts is at the expense of time. And once there is a drop, even if it is just a few seconds, it may cause the entire production to stop or even equipment, personal safety accidents.
l Reliability
Since Ethernet was designed at the beginning, it was not based on industrial network applications. When it is applied to an industrial site, in the face of adverse conditions, severe line-to-line interference, etc., these will inevitably cause its reliability to decrease. Industrial networks must have good reliability, recoverability, and maintainability in a production environment. That is, to ensure that any component in a network system fails, it will not cause crashes and embarrassment of applications, operating systems, and even network systems.
l Ethernet industry application solution mechanism
In view of the three major defects of Ethernet and the special requirements of industrial fields for industrial networks, various methods have been adopted to improve the performance and quality of Ethernet to meet the requirements of the industrial field. Here are some solutions:
l Exchange technology
In order to improve network congestion when the Ethernet load is heavy, an Ethernet switch can be used. It uses an efficient collision domain demarcation technology that will share the LAN. Each conflict domain is connected by a switch to reduce the conflict problems and error transmission caused by the CSMA/CD mechanism. In this way, conflict can be avoided as much as possible and the system's certainty can be improved. However, this method has a high cost and there is a certain delay in the allocation and buffering process.
l Fast Ethernet
We know that the greater the load on the network, the greater the rate of conflict. There is data showing that when a network's negatives are below 36%, there is basically no conflict. When the load is below 10%, the 10M Ethernet collision rate is once every five years. The 100M Ethernet collision rate is once every 15 years. However, after more than 36%, the rate of conflict with the increase in load increases at a geometric progression rate. Obviously improving the Ethernet communication speed can effectively reduce the network load. Fortunately, Ethernet now has a communication rate of 100M/S and 1G/S high-speed Ethernet. With careful and comprehensive design and control of the number of network nodes and traffic in the system, Ethernet can be used. Web as an industrial network.
l IEEE1588 timing mechanism
IEEE 1588 defines a precise synchronous clock protocol (PTP) related to network communication, local computing, and allocation of objects in measurement and control networks. This protocol is not exclusive, but it is particularly suitable for Ethernet-based technologies with an accuracy in the microsecond range. It uses a time stamp to synchronize the local time mechanism. Even if the synchronization control signal produces certain fluctuations in the network communication, the accuracy it achieves can still meet the requirements. This makes it particularly suitable for Ethernet-based systems.
By adopting this technology, the Ethernet TCP/IP protocol can operate in high-precision network control systems without major changes. In the area bus it achieves far more precision than existing systems. In addition, the use of Ethernet-based TCP/IP-based network technology at all levels of the enterprise has tremendous advantages.
A simple system that includes the IEEE 1588 timing mechanism includes at least one master clock and multiple slave clocks. If there are multiple potential master clocks at the same time, the active master clock will be determined based on the optimized master clock algorithm. All clocks constantly compare the clock attributes with the master clock. If a new clock is added to the system or the existing master clock is disconnected from the network, other clocks will re-determine the master clock. If multiple PTP subsystems need to be interconnected, they must be implemented by the boundary clock. A port of the boundary clock is connected to the subsystem as a slave port and provides the clock standard for the entire system. Therefore, the main clock of this subsystem is the original master clock of the entire system. The other ports of the boundary clock serve as the master port, and the synchronization information is transmitted to the subsystem through these ports of the boundary clock. The port of the boundary clock is an ordinary clock to the subsystem.
The precise network synchronization protocol defined by IEEE1588 achieves a high degree of synchronization in the network, so that no special synchronous communication is required when distributing control work, thereby achieving the effect that the communication time mode is separated from the application execution time mode. Due to the high-precision synchronization work, the data transmission time fluctuation inherent in the Ethernet technology is reduced to an acceptable level without affecting the range of control accuracy. One of the great advantages of IEEE1588 is that its standards are very representative and open-ended. Because of its openness, many suppliers of control systems have now applied the standard to their products. And manufacturers of different devices follow the same standards, so that their products can also ensure good synchronization.
l Typical Industrial Ethernet
With the rapid development of Ethernet technology and its 80% market share and obvious defects of fieldbus, major manufacturers in the industrial control field have been developing industrial Ethernet products that are compatible with their industrial control products and have strong compatibility. One of the most widely used industrial Ethernets is the SIMATIC NET Industrial Ethernet developed by the German Siemens company. It provides open, different communication systems for various levels of control in industrial environments. These communication systems are based on national and international standards and conform to the ISO/OSI network reference model. The main architecture of SIMATIC NET Industrial Ethernet consists of network hardware, network components, topology, traffic processor and SIMATIC NET software.
l SIMATIC NET Industrial Ethernet basic types and network hardware
There are two types of SIMATIC NET Industrial Ethernet, 10Mbit/s Industrial Ethernet and 100Mbit/s Industrial Ethernet. It is a cell-level and control-level transmission network that utilizes the band transfer technology based on IEEE 802.3 using CSMA/CD media access methods. In Siemens Industrial Ethernet, the commonly used physical transmission media are shielded twisted pair (TP), industrial shielded twisted pair (ITP), and fiber optics. TP connections are commonly used for end-to-end connections. A data terminal equipment (DTE) is directly connected to the network connection component port, and the device is responsible for amplifying and forwarding the signal. In SIMATIC NET Industrial Ethernet, these network connection elements are OLM (Optical Link Template) ELM (Electrical Connection Template) OSM (Optical Switch Template) and ESM (Electrical Switch Template). The DTE is connected to the connecting element via a TP or ITP cable.
l SIMATIC NET Industrial Ethernet network components
SIMATIC NET Industrial Ethernet network components include Industrial Ethernet link modules OLM, ELM and Industrial Ethernet switches OSM/ESM and ELS, and Industrial Ethernet Link Modules OMC. Among them OLM (optical link module) has 3 ITP interfaces and two BFOC interfaces. The ITP interface can connect three terminal devices and network segments. The BFOC interface can connect two optical path devices (such as OLM, etc.) at a speed of 10 Mbit/s. The ELM (Electrical Link Module) has 3 ITP interfaces and one AUI interface. Through the AUI interface, network devices can be connected to the LAN at a speed of 10 Mbit/s. On an ordinary OSM, the electrical interface (TP/ITP) is 10/100 Mbit/s adaptive and the line sequence is adaptive. The fiber interface is a 100 Mbit/s full-duplex BFOC interface for multi-mode fiber connections. The maximum distance between two OSMs is 3km. Up to 50 OSMs can be connected on the same network segment. The extended distance is 150 km. At the same time, it also has address learning, address deletion, setting transmission baud rate (10 or 100 Mbit/s) and adaptive function, simplifying network configuration and enhancing network expansion capability. In addition, according to the IEEE 802.1Q standard, OSM/ESM also supports VLAN (Virtual Local Area Network), which provides VLAN priority labels for packets. It assigns data as a low-to-high (0-7) priority level, and data packets with no destination address are treated as low-priority data frames.
l Topology of SIMATIC NET Industrial Ethernet
l Bus topology
In the OLM or ELM bus topology, DTE devices can be connected to OLMs or ELMs via ITP cables and interfaces. There are three ITP interfaces for each OLM or ELM. The OLMs can be connected by optical cables. Up to 11 cascades can be connected. The ELM can be connected through ITP XP standard cables, and up to 13 cascades can be connected. ESM can be connected via TP/ITP cable to form a general network. Any port can be used as a cascaded port. The distance between two ESMs cannot exceed 100m, and the entire network can connect up to 50 ESMs.
l Ring topology
The OLM can connect the bus network end to end through fiber optic cable to form a ring network. Up to 11 OLMs can be cascaded across the entire network. Compared with a bus network, redundant ring networks increase the reliability of data exchange. The OSM/ESM can also form a ring topology. They have network redundancy management capabilities. They can use the DIP switch to set up any OSM/ESM in the network as a redundancy manager. Therefore, a redundant ring network can be formed. Among them, the 7 and 8 ports on the OSM/ESM are used as the optical fiber cable level connection ports of the ring network. The OSM as a redundancy manager monitors the state of ports 7 and 8. Once a network outage is detected, the entire network will be rebuilt and the network will be switched to the backup channel to ensure that the data exchange will not be interrupted. The network reconstruction time is less than 0.3S.
l Ring redundancy
In Siemens Industrial Ethernet, there is a standby-sync interface on each OSM/ESM (except OSM TP22 and ESM TP40). Use a pair of OSMs/ESMs to set the standby and standby slaves via the DIP switches. With the use of ITP XP standard cables to connect the standby interfaces, the pair of OSMs/ESMs can be used to redundantly connect to another ring. The standby master and slave are connected via ITP XP9/9 standard cables. When the standby master channel fails, the standby slave connection channel works. When the standby master channel returns to normal, the standby master informs the standby slave that the standby slave will stop working. The entire network reconstruction time is less than 0.3m.
l SIMATIC NET Industrial Ethernet Communication Processor
Commonly used SIMATIC NET Industrial Ethernet Communication Processors (CPs), including the CP243-1 series, CP343-1 series, CP443-1 series, and network cards used on PCs, which are used on S7 PLC stations, and provide ITP, RJ45 Ethernet interfaces such as AUI. They connect a PLC or PC to Industrial Ethernet at 10/100 Mbit/s. CP series templates are used for communication of S7 series PLCs in the composition of industrial Ethernet. Users can easily connect S7 series PLCs via Ethernet through the CP series of templates and support the use of STEP7-Micro/WIN32 software. The network remotely configures, programs, and diagnoses S7-series PLCs. At the same time, Ethernet can be connected between PLCs in the S7 series and can also communicate with the OPC server on the PC.
l SIMATIC NET Industrial Ethernet Software
The SIMATIC NET Industrial Ethernet software includes SIMATIC NET V6.2 and OPC (OLE for Process Control), where the SIMATIC NET software improves the unified Windows session and also integrates and updates more functions, in particular it provides APC. (Advanced PC Configuration) Advanced PC Configuration Tool. Through the configuration of APC, the PC can be used as a site of the entire system control system to communicate with other PLC stations. It also provides OPC Server and data processing functions. OLE (object connection and embedded) itself is an application based on Microsoft COM technology, and OPC interface is an open unified software interface based on OLE. OPC does not depend on a certain manufacturer. Almost all industrial control software and hardware controllers have integrated OPC interfaces. Therefore, device communication between different hardware vendors can be performed through a unified OPC interface, thereby avoiding the need for manufacturers of different devices. Differences in communication protocols cause difficulties in data exchange. SIMATIC NET OPC is a server/client structure. The program interface of the client access server has automation interfaces and user-defined interfaces. Only custom interfaces can be used to access fault alarms and trigger event messages. The structure of SIMATIC NET OPC is a hierarchical mode, ie OPC server -- OPC group -- OPC item. OPC data access is based on this structure.
l Prospects and Prospects of Industrial Ethernet
Industrial Ethernet attracts more and more manufacturing manufacturers with its unique low-cost, high-efficiency, high-expansion, and high-intelligence charm. On the one hand, as many manufacturers develop and develop industrial Ethernet technologies, if they are not unified, as in the case of fieldbus, there are many standards and poor compatibility, which in turn affects the development of industrial Ethernet. Just like this, the international community has begun to develop an industrial Ethernet standard. At the third meeting of the International Industrial Ethernet Series Drafting Working Group (IEC/SC65C/WGs) held at 2004.5 Beijing, we have seen an emerging international standard for industrial Ethernet. The series of standards will be held at 2005.8. The final draft was officially released in the second half of 2007 after the request for secondary opinions of 2006.2 and 2006.12. Make this series of standards from the IS standard to IEC standard 6. On the other hand, the rapid development of Ethernet and communication technologies has also led to the further development of industrial Ethernet technology. Industrial Ethernet technology has now begun to develop in the direction of real-time industrial Ethernet and wireless industrial Ethernet. In particular, Austrian B&R has developed a truly real-time Ethernet (Ethernent Powerlink), and in the near future, a new generation of industrial Ethernet components for future industrial networks will emerge. Because Ethernet has the reputation of “one network after another,†it can be extended to the enterprise site.